A diesel-exhaust-fluid is typically an aqueous solution of urea that is used in a process called selective catalytic reduction (SCR) to reduce emissions of oxides of nitrogen (NOX) from the exhaust of diesel engines employed in motor vehicles. Although the urea solution employed for such purposes generally contains high-purity urea dissolved in de-mineralized water and is non-toxic and safe to handle, the solution can be corrosive to some metals. Accordingly, such a urea solution must be stored and transported using containers that are manufactured from appropriate inert materials.
An SCR-equipped vehicle typically carries its urea solution onboard in a specially designed reservoir. The urea solution is dosed into the SCR system during engine operation at a rate equivalent to 3-5% of consumption of the diesel fuel. Such a low dosing rate ensures significant periods between refills and minimizes the reservoir's impact on chassis space. On-highway SCR systems are currently in use throughout Europe, in Japan, Australia, Hong Kong, Taiwan, Korea, New Zealand and Singapore. The United States Environmental Protection Agency (US EPA) will limit NOX by a legislative enactment to levels that will require North American trucks to be equipped with SCR post-2010.